1. Technical Field
The present invention relates to image forming apparatuses and image forming methods.
2. Related Art
Ink jet printers are exemplary image forming apparatuses and record or print an image on an arbitrary recording medium, such as a paper sheet or a film, by ejecting ink droplets onto the recording medium to create a plurality of dots thereon. An exemplary ink jet printer performs a dot creating operation (pass) and a transport operation; in the dot creating operation, the head ejects ink droplets onto a recording medium through a plurality of nozzles formed in the head while moving over or scanning the medium in a main-scanning direction, thereby creating a dot array (also called a raster line) extending in a main-scanning direction, and in the transport operation, the head moves or is transported in a sub-scanning direction that intersects the main-scanning direction. Repeating the dot creating and transport operations creates dots arranged densely on the recording medium in both the main-scanning and sub-scanning directions, and these dots constitute an image.
To record high-quality images, some ink jet printers form a raster line on a medium through multiple passes while transporting the medium in the sub-scanning direction by an amount smaller than the width of the head. For example, JP-A-2010-17976 proposes an image forming method of printing an image, in which a print region is divided into sub-regions depending on an image to be recorded onto a recording medium and these sub-regions are scanned a different number of times.
When printing an image through multiple passes, an ink jet printer, as described above, creates different numbers of dots through respective nozzles arrayed in the sub-scanning direction by changing the numbers of ink droplets to be ejected from the nozzles. Unfortunately, if different numbers of ink droplets are ejected from the nozzles, the volume of the ink droplets ejected may differ. Thus, different sized dots may be created. Consequently, an image with prominent density nonuniformity, namely, a low-quality image might be recorded.